Weight gain is not always just a matter of lacking willpower, but has more to do with how your brain reacts to what it sees, according to a new study by neuroscientists.

For some people, just looking at tasty images of food is enough to make them want to eat.

Individuals who are more susceptible to cravings after smelling or seeing food — even if they aren’t hungry — have what researchers call “external food sensitivity,” or EFS. In the new study, conducted by scientists at the British Medical Research Council, brain scans revealed how this food sensitivity influences people's eating habits.

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The researchers Andrew Calder, Luca Passamonti and James Rowe were trying to determine why some people are more likely to overeat. What they found was, “people who appear to be more sensitive to food signals have different wiring in their brains,” said obesity expert Marc-Andre Cornier, M.D., a University of Colorado endocrinologist who was not associated with the trial.

In the small study, just published in the January Journal of Neuroscience, brain scans showed how 21 participants reacted to three sets of images: appetizing food, bland food, and unrelated images of other subjects. The volunteers were of normal weight because obese people have already undergone “neurohormonal changes” that affect how their brains function, according to researcher Andrew Calder.

The participants were quizzed to determine their susceptibility to food cues. Then they fasted for two hours. At that point, their level of hunger was assessed. When researchers checked the brain scans, the results were clear.

Craving a rewardBefore viewing the food images, those with a high food sensitivity rating didn't report being any hungrier than subjects with a normal rating. But after seeing the images, the higher the food sensitivity rating, the hungrier the test subjects felt. So, researchers concluded that the images triggered feelings of hunger in susceptible people.

Such differences in wiring help explain why some people eat even if they aren’t hungry. An external cue, like the sight of tempting food, triggers a desire for a reward.

“We don't think it’s overstating matters to say that chronic overeating could … be considered an ‘addiction’ for food,” said Calder.

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In fact, such a craving for a reward is analogous to that found in the brains of drug addicts and many smokers, said Rexford Ahima, a University of Pennsylvania, Philadelphia, researcher who studies the neural circuitry involved in feeding.

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The data from the brain scans showed that the “connectivity” between regions of the brain related to eating, like the amygdala and the ventral striatum, was different in the subjects with high EFS.

“Connectivity is a measure of ‘synchrony’ between different brain regions,” Calder explained. “In this case, we are suggesting that the brain areas implicated in eating and coding the reward value of foods show less synchrony in high EFS individuals,” meaning that the timing of signals between these areas is off.

So, roughly stated, instead of sparking real hunger, the signals make people crave the reward and then think they are hungry.

Regulate food marketingIf more research confirms these findings, experts said, the results could point the way to effective drug or behavioral interventions to prevent or treat obesity.

The study provides further confirmation that chronic overeating isn’t always as easily controlled as going on the latest fad diet, explained Ahima. The people aren't even looking at real food, “and look at how their brains light up.”

Of course, not all people with this wiring become obese. But such people, given the right environmental cues, will find it much tougher to resist.

Calder suggested that his research validates concerns about the ways high-calorie and high-fat foods are marketed and advertised. Though banning Mrs. Butterworth from TV, as was done with cigarettes is unlikely, Calder and other researchers suggest some kind of government regulation of food advertising could ease the spreading obesity epidemic.

More likely, science will turn to new interventions to fix the problem. “The brain is plastic,” Cornier said. “It can change.”